Support more complicated path_regexp

Is it possible to support more complicated regexp in path_regex? Like mentioned in the documentation here https://golang.org/pkg/regexp/#MatchString

"More complicated queries need to use Compile and the full Regexp interface"

so the match has to be implemented more like this

validRegExp = regexp.MustCompile(path_regexp)
match = validRegExp.MatchString(filePath))

I think this is the reason for issues like this one https://github.com/mozilla/sops/issues/480

We also had problems to specify a path which is including the filename and not just a filename. Especially if there are '/' in the path it's not working at all and we get the message 'no matching creation rules found'

Example not working (no matter if '/' are escaped or not!):

path_regexp: stages/dev/secrets.env$

Examples working:

path_regexp: .*dev.*
path_regexp: secrets\.env$

Trace how the path_regexp creation rule is evaluated against file paths, using the reported stages/dev/secrets.env$ and secrets.env$ examples as reproductions. Check the existing matching behavior against Go's regexp documentation; done means valid expressions match paths containing directories and no matching creation-rules error occurs for the reported case.